Super high frequency attenuator



June 8, 1948. E. G. LlNDER 2,443,109

v SUPER HIGH FREQUENCY ATTENUATOR Filed May l, 1945 Gtorneg PatentedJune S, 1048 SUPERl HIGH FRE QUENGY ATTENUATOR lErnest G. Linder,Princeton, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application May l, 1943, Serial No. 485,357v

(Cl. 178r- 44.)

2 Claims. 1

invention relates generally to super-high frequency attenuators, andmore particularly to attenuators of a type employing semi-conductiveplastic materials as dielectrics in the super-high freflllIlC/y eld. Theterm super-high frequency isused to Icover approximately 3,000,000kilocycles to 30,009,000 kiloeyeies. The invention provides `an,efficient and economical means for obtaining predetermined attenuation`of Vsuperhigh frequency energy in wave guides or concentric.transmission lines, as well as an effective means for preventingleakage of super-high frequency energy through openings in shieldingenclosures therefor.

A convenient form of attenuator for a wave guide vvcomprises a shortsection of concentric line inserted within the guide. The line shouldpreferably include ycoupling loops at each end thereof tov providesuitable coupling and impedance matching to the wave guide.

The concentric line section preferably includes a central conductorseparated from the vouter concentric conductor by .a dielectriccomprising energy absorptive material such, lfor example, as conductiverubber oi relatively low D.C. resistivity.A High direct currentinsulation may be provided conveniently by separating the high lossdielectric from the outer conductor by a relatively thm layer ofvarnished cambric or other low conductivitymateral.

The attenuation characteristics of `a lter of the type .describedheretofore may be readily calculated for energy absorptive materials ofknown D.C resistivity and dielectric constant. The specific super-highfrequency resistivity of the particular energy absorptive materialemployed maybe computed from the propagation function.

v=\/ R+jwL G+jwc 1) where R is the series resistance, L is the `seriesinductance, G is the shunt conductance, and C is the shunt capacitance,all per unit length. In the present case R is always negligibly small,and.

in the case of the lower resistivity dielectric materials, I@also issmall compared to G. Hence,

where a is the attenuation and is the phase constant. Since this becomeswhere the units are in the MKS system, a and b are the inner and outerradii, respectively, and fr is the permeability. Also Inserting theseexpressions in (2) yields a- Zp n (3) This-is identical with theexpression for the attenuatiQn. 0f a plane Wave in a medium osufiiciently high conductivity so that Pea) where e is the permittivity(see Slater, Microwave Transmission, page 11,1). In the present casethis condition is adequately met. Hence, Formula 3 may be applied hereto both the concentric line andwave guide measurements.

The same calculations apply to an attenuator comprising a simple plugorl diaphragm of energy absorptive material inserted Within a wave guideusing the H01 mode of transmission.

For an energy absorption material comprising a conductive syntheticrubber such as neoprene, having sufficient carbon content, such asacetylene blaclctol provide a D.C, resistivity of the order of 5ohm-centimeters, a convenient concentric line type filter may beconstructed to provide an attenuation ot the order of 'decibels percentimeter, and to provide a D.C. insulation effectiye for at least172,000 volts. By way of example, the diameter of the outer conductormasl be approximately .6 centimeter while that of the inner conductormay be ofthe order of .l centi'- ,meten A typical formula for conductiverubber having a resistivity of the order of ohm-centimeters is:

Cured 45/290 F.

If an energy absorptive material of the same general type, but having aD.C. resistivity of the order of 200 ohm-centimeters, is employed, anattenuation of the order of 3 decibels per centimeter may be readilyprovided with a line having an outer conductor diameter of .9 centimeterand an inner conductor diameter of .15 centimeter. The foregoingcalculations vare based upon the assumption that the input and outputelements of the attenuator are properly matched to the Wave guide. Inactual practice, such filter sections Would probably be mismatchedintentionally, thereby providing attenuations higher than the calculatedvalues.

Such filters may be employed for providing effective attenuation ofsuper-high frequency energy in power leads to super-high frequencyapparatus, by utilizing the power lead as the central conductor of ashort section of concert; tric line which includes energy absorptivematerial of the type described heretofore. Similarly, the energyabsorptive material, such as the conductive synthetic rubber describedheretofore, may be employed in the form of gaskets to prevent leakage ofsuper-high frequency energy through openings in shielding enclosures.

Among the objects of the invention is to provide an improved method ofand means for attenuating super-high frequency energy. Another object isto provide an improved method of and means for preventing leakage ofsuperhigh frequency energy through openings in shielding enclosurestherefor. A further object is to provide an improved attenuator forsuperhigh frequency energy in power leads to superhigh frequencyapparatus. Another object is to provide an improved super-high frequencyattenuator comprising a concentric line section which includes a.dielectric of relatively low D.C.

resistivity and relatively high energy absorptive characteristics atsuper-high frequencies. An-l other object of the invention is to providean im proved attenuator fon super-high vfrequencies which providesrelatively uniform attenuation over a wide band of frequencies. A still`further object of the invention is to provide an improved means forsealing apertures or joints in superhigh frequency shielding enclosures.

The invention will be described in further detail by reference to theaccompanying drawing, of which Figure 1 is a cross-sectional view of oneembodiment of the invention, Figure 2 is across-sectional view of asecond embodiment thereof, Figure 3 is the section III-III of Fig. 2rFigure 4 is a family of graphs illustrativeV of the invention, andFigures 5 and 6 are VI'nodii'ications. of a third embodiment of theinvention. Similar' reference numerals. are applied to similar elementsthroughout the drawing.

Fig. 1 includes a wave guide I having metallic aperture devices 3, 5transversely disposed there for example, as varnished cambric.

giriamo 4 in and separated axially a distance predetermined by thecalculated length of the attenuator. A cylindrical outer conductor 1connects the aper tures of the conductive aperture devices 3. 5 and issupported thereby substantially coaxially with the axis of the waveguide I. An inner conductor 9 is disposed substantially coaxially withthe outer conductor I and is separated therefrom by an energy absorptivedielectric II and a relatively thin layer of D.C. insulating material I3such, The ends of the inner conductor 9 are terminated in coupling loopsI5, I1 which are connected, respectively, to the ends of the outercylindrical conductor l. If desired, the characteristics of the couplingloops I5, I 'i may be selected to provide desired impedance matchingwith the wave guide I at the operating frequency. The lengths anddiameters of the inner and outer conductors 9, l, respectively, and thecharacteristics of the dielectric materials I I, I3 may be readilycalculated from the formulae discussed heretofore, to prof vide desiredattenuation at the selected operat-v ing frequency. Figs. 2 and 3illustrate attenuators of the general type described in connection withFig. f1, which may be employed to attenuate effectively super-highfrequency currents on, for example, a power conductor 9 Which'enters ashielding enclosure I9. The concentric line section consists of thepower conductor 9 and an outer cylindrical conductor l, separated by anenergy absorptive dielectric II and a D.'C. insulating dielectric I3.The outer cylindrical conductor 'I Vmay be fastened to the shieldingenclosure I9 by means 'of brackets 2i which are preferably soldered tothe cylindrical conductor 'i and the shielding `enclosure I 9.

Optimum values of resistivity will obtain 'for filters of predeterminedproportions and char.-

acteristics. follows:

If the resistivity of the semi-conductive rubber is sufficiently lowthat considerable current liowsl longitudinally on its surface, it maythen be regarded as the inner conductor of the transmission line. Thiscondition obtains when the ef#l f-ective skin thickness becomes lessthan the radius of the rubber. v regarded as a low impedance line havinga good dielectric (egg, the varnished cambric insula- 'Ihis feature maybe explained as tion). The attenuation a2 may be determined by theconventional formula: p

I-x/#wk Y l where 1c is the dielectric constant of the varnishedcambric, a=41r109 henries per centimeter, w=21rf, p is the resistivity,and :r is the thickness I of the varnished cambric or other insulation.

If the resistivity of the rubber is high, so thatv The lter may then belFig. 4 shows theoretical curves of the relations between attenuation andresistivity in filters of the type described which employ threedifferent thicknesses of varnished cambric insulation having adielectric constant equal to 4.

Fig. 5 illustrates a convenient method of employing energy absorptivematerial to prevent leakage of super-high frequency energy around theedges of a door in a shielding enclosure for super-high frequencyapparatus. An enclosure I9 is provided with a door or cover 29 having atapered edge 23 which ts a complementarily tapered ja-mb 25 at theaperture of the shielding enclosure I9. Energy absorptive material 21 isinserted in the aperture between the tapered portions 23, 25 of the doorand jamb, respectively. Due to the tapered form of the door and jarnb,the effective length of the attenuator may be relatively large incomparison to the cross-sectional area of the aperture in the shieldingenclosure.

A bolt 29, fastened to the shielding enclosure I9, extends through ahole in the door 20. The door 20 may be secured in a closed position bymeans of a nut 3| threaded to the end of the bolt 29 extending throughthe door. As many bolts and nuts as desired may be disposed at regularintervals around the edge of the door. Due to the relatively long energyabsorbing path in the dielectric 21, between the tapered portions 23,

25 of the door an-d jamb, there will be relatively low energy leakagealong the bolt 29 between the door 20 and the jamb 25. Preferably alljoints between the tapered portion 25 and the body of the shieldingenclosure I9 should be soldered to eliminate energiT leakage around thegasket.

A modification of the super-high frequency gasket is illustrated in Fig.6. In this arrangement a shielding enclosure I9 includes a door frame 26extending wel] into the door opening in the enclosure I9. The door 29abuts against the door frame 26 and is separated therefrom by a gasketof super-high frequency energy absorptive material 21. A bolt 29 issecured to the door frame 26 and extends through a suitable aperture inth'e door. The door may be retained in a closed position by means of anut 3| threaded to the portion of the bolt 29 extending through thedoor. This modification has the distinct advantage that the door frame26 must necessarily extend well into the door aperture in order toprovide a gasket as effective as that described for the tapered doorframe of the modification shown in Fig. 5. However, where space is notan important consideration, either modification may be employed toadvantage.

It should be understood that the highly conductive rubber employed as adielectric or energy absorber in the various super-high frequencyattenuators described heretofore may be similarly employed to preventleakage of super-high frequency energy around rotating or movable shaftsextending into various other types of enclosures. Various modificationsand embodiments thereof are disclosed and claimed in a copending U. S.application of George L. Fernsler, Serial Number 485,012, filed April29, 1943, assigned to the same assignee.

I claim as my invention:

1. An attenuator for supeiwhigh frequency energy comprising atransmission line including two conductors, a leakage path for saidenergy including e. first dielectric having a predetermined proportionof energy absorptive material, and a second dielectric interposedbetween said first di electric and one of the conductors of said line,characterized by the relation Ee p k where p is the resistivity of thefirst dielectric, x is the thickness of the second dielectric, and lc isthe dielectric constant of the second dielectric.

2. An attenuator for super-high frequency energy including apredetermined length of coaxial transmission line having a rst soliddielectric including a predetermined proportion of energy absorptivematerial, a second dielectric interposed between said first dielectricand one of the conductors of said line, the resistivity of said firstdielectric being a value just sufficiently high effectively to preventsubstantial current skin effect thereon adjacent to the surface of saidsecond dielectric in contact therewith, and a pair of coupling loopseach terminating a different one of th'e ends of said line.

ERNEST G. LINDER.

REFERENCES CITED The following references are of record in the f le ofthis patent:

UNLTED STATES PATENTS Number Name Date 2,238,915 Peters et al Apr. 22,1941 2,283,895 Mouromt'seif et al. May 19, 1942 2,304,210 Scott et alDec, 8, 1942 2,322,773 Peters June 27, 1943 2,409,640 Moles Oct. 22,1946 FOREIGN PATENTS Number Country Date 456,722 Great Britain Nov. 11,1936 526,895 Great Britain Sept. 27, 1940

